Metal oxide semiconductor field effect transistors (typically referred to as MOSFETs) are capable of the high switching speeds needed for powering such loads as an electric motor from a DC supply. However, the current rating of available MOSFETs is not adequate to carry the current required for many sizes of motors. Accordingly, it is often desirable to operate two or more power MOSFETs in parallel in a switching circuit.
The forward resistance of the drain-source circuit of power MOSFETs is typically uniform from MOSFET to MOSFET. Accordingly, the MOSFETs may be connected in parallel in a switching circuit and the switched currents will divide substantially equally among the MOSFETs so that the individual MOSFET current rating is not exceeded.
A power MOSFET has an inherent reverse diode junction between the source and drain elements. When the MOSFET switch is used with a regenerative load, the reverse current which occurs as the switch is opened or turned off flows through the reverse diode junction. The resistance of the reverse junctions of MOSFETs may differ substantially. With a plurality of MOSFETs connected in parallel, the reverse current from a regenerative load divide in inverse relation to the reverse junction resistances. The current may exceed the reverse junction current rating, causing MOSFET failure.